\begin{figure}[H]
  \begin{center}
    \unitlength=4pt
    \begin{picture}(25, 14)(0,-4)
    \gasset{Nw=5,Nh=5,Nmr=2.5,curvedepth=3}
    \thinlines
    \node[Nmarks=if,iangle=180,fangle=180](A1)(0,0){$1$}
    \node(A3)(25,0){$3$}
    \drawloop[loopangle=90](A1){$a^2+b^2$}
    \drawloop[loopangle=90](A3){$a^2+b^2$}
    \gasset{curvedepth=4}
    \drawedge(A1,A3){$ab+ba$}
    \drawedge(A3,A1){$ab+ba$}
    \end{picture}
  \end{center}
  \caption{Après élimination de l'état $4$.}
\end{figure}